Remove the ability to bring your own flatc

[jathu]

We currently build flatc from source, however we also:

• Allow people to bring their own flatc via the FLATC_EXECUTABLE flag
• Install flatc from pip for some builds

Given we rely on flatbuffers headers from the one we import, I think we should make this completely hermetic and always build flatc from source.

What are some reasons to allow bringing in flatc?

cc @larryliu0820 @lucylq

[tdakhran]

cross-compilation is one of the reasons, we use FLATC_EXECUTABLE to supply flatc precompiled for host system.

[jathu]

@tdakhran that should be fixed once this lands #9077

[jathu]

We now build flatc for the host on all platforms: #9077, #9306.

After some thought, I don't want to remove the ability for the user to set FLATC_EXECUTABLE since it provides an escape hatch.